Oxygen is produced commercially in large quantities by the cryogenic rectification of feed air, generally employing the well known double column system, wherein product oxygen is taken from the lower pressure column. At times it may be desirable to produce oxygen at a pressure which exceeds its pressure when taken from the lower pressure column. In such instances, gaseous oxygen may be compressed to the desired pressure. However, it is generally preferable, both for safety and for operating cost purposes, to remove oxygen as liquid from the lower pressure column, pump it to a higher pressure, and then vaporize the pressurized liquid oxygen to produce the desired elevated pressure product oxygen gas.
Cryogenic rectification requires refrigeration in order to operate. The requisite refrigeration is increased when oxygen is withdrawn from the column as liquid and pumped prior to vaporization because the pump work is added to the system. Refrigeration may be provided to the cryogenic process by the turboexpansion of a stream fed into the rectification column system. However, the compression of a stream for the turboexpansion consumes a significant amount of energy.
Accordingly, it is an object of this invention to provide a cryogenic rectification system wherein liquid oxygen is removed from the column system for boiling and wherein process refrigeration is provided by turboexpansion of a feed stream, which has improved operating efficiency over conventional oxygen product boiler cycles.